172 CLINICAL APPLIED ANATOMY. 



of the vital centres of the bulb to the vertebral and basilar arteries, 

 thrombosis of these vessels is a more serious menace to life than 

 occlusion of the middle or posterior cerebral trunks. Thrombosis 

 of cortical branches may produce slight or transient symptoms, 

 since there is usually a fairly free anastomosis of vessels in the 

 pia mater. Sometimes the vascular disease at the base is so 

 situated as to occlude the perforating branches without materially 

 obstructing the main trunk, softening of the pons Varolii, and of 

 the central ganglia of the brain may be brought about in this 

 way. 



The grey matter of the brain is much more richly supplied with 

 vessels than is the white matter, hence softening in the grey 

 substance has a red appearance in its earlier stages, whilst in the 

 white substance the red coloration is wanting. Thrombosis in 

 a main trunk may spread by continuity to the branches which 

 the trunk gives off. Thus a clot in the internal carotid artery may 

 spread into the anterior and middle cerebral vessels or into the 

 ophthalmic artery. 



It is obvious that thrombosis of the middle cerebral artery can 

 produce hemiplegia from either capsular or cortical softening, 

 since it supplies the former by perforating and the latter by 

 cortical branches. Aphasia may result from the cortical softening, 

 and such softening is nearly always thrombotic, for in embolism 

 of the same trunk the healthy anastomosing vessels aided by the 

 higher blood pressure are usually sufficient to carry on the 

 circulation. In the ordinary type of hemiplegia the leg suffers 

 less than the arm and recovers more rapidly, but when the focus 

 of softening is in the posterior part of the capsule producing 

 hemianaesthesia and hemianopia, the motor weakness may be 

 expected to be most marked in the lower limb, since the motor 

 fibres of the leg lie nearer the sensory portion of the capsule than 

 do the motor fibres for other parts of the body. (Fig. 12, p. 179.) 



The area of distribution of the posterior cerebral artery has 

 already been indicated in the section on cerebral embolism. 



Symptoms pointing to occlusion of the basilar artery are almost 

 always due to thrombosis, for the anatomical conditions here do 



